The present invention relates to the manufacture of glass fiber pellets. In particular, the present invention provides an apparatus and process for making polymer coated glass fiber pellets by combining multiple segments of a chopped multi-fiber glass strand into pellets and encapsulating the pellets in a polymer shell. Such pellets provide a convenient form for the storage and handling of chopped glass fibers used as reinforcing materials in composite structures.
Chopped glass fibers are commonly used as reinforcement materials in thermoplastic articles. Typically, such fibers are formed by drawing molten glass into filaments through a bushing or orifice plate, applying a sizing composition containing lubricants, coupling agents and film-forming binder resins to the filaments, gathering the filaments into strands, chopping the fiber strands into segments of the desired length, and drying the sizing composition. These chopped strand segments are thereafter mixed with a polymeric resin, and the mixture supplied to a compression-or injection-molding machine to be formed into glass fiber reinforced plastic articles. Typically, the chopped strands are mixed with pellets of a thermoplastic polymer, and the mixture supplied to an extruder wherein the resin is melted, the integrity of the glass fiber strands is destroyed and the fibers are dispersed throughout the molten resin, and the fiber/resin dispersion is formed into pellets. These pellets are then fed to the molding machine and formed into molded articles having a substantially homogeneous dispersion of the glass fibers throughout.
Unfortunately, however, chopped glass fibers made via such processes are typically bulky and do not flow well. Consequently, such fibers are difficult to handle and have been problematic in automated processing equipment.
One attempt at solving this problem has been to compact the chopped strands into denser rod-shaped bundles or pellets to improve the flowability of the chopped strands, and enable the use of automated equipment to weigh and transport the glass fibers for mixing with the thermoplastic resins. Such a process is disclosed in U.S. Pat. No. 4,840,755, wherein wet chopped strands are rolled, preferably on a vibrating carrier, to round the strands and compact them into denser, cylindrically shaped pellets. While such methods and apparatus tend to provide denser, more cylindrically shaped pellets exhibiting better flowability, they are undesirably limited in certain respects.
For example, the pellet size and fiber content are generally limited by the size and number of fibers in the chopped strand, in that the process is designed to avoid multiple chopped strand segments from adhering together to form pellets containing more fibers than are present in a single chopped strand. Consequently, to obtain pellets having a suitable bulk density and a sufficient ratio of diameter to length to exhibit good flowability, the strand from which the segments are chopped usually must be formed of a large number of filaments. However, increasing the number of filaments required to be formed and combined into a single strand undesirably complicates the forming operation.
In an attempt to overcome these shortcomings, U.S. Pat. No. 5,578,535 discloses glass fiber pellets that are from about 20 to 30 percent denser than the individual glass strands from which they are made, and from about 5 to 15 times larger in diameter. These pellets are prepared by hydrating cut strand segments to a level sufficient to prevent filamentization but insufficient to cause the strand segments to agglomerate into a clump, and mixing the hydrated strand segments for a time sufficient to form pellets. Suitable mixing includes a process that will keep the fibers moving over and around one another, such as tumbling, agitating, blending, commingling, stirring and intermingling.
Although the disclosed pellets can be made by such diverse mixing processes, it has been discovered that many of such processes are either too inefficient to be used commercially, or cannot be adequately controlled to produce a uniform pellet product that provides the resulting composite article with strength characteristics comparable to those made from nonpelleted chopped strand fibers. For example, the use of a modified disk pelletizer frequently results in excessive residence time of the formed pellets within the mixer, which in turn results in degradation of the pellets due to the abrasive nature of glass fiber pellets rubbing against one another. Such pellet degradation ultimately reduces the strength characteristics of the molded articles made therewith.
However, an efficient pellet-forming process and apparatus that controllably yields a uniform glass fiber pellet product that provides strength characteristics equal to nonpelleted chopped strand fibers in composite molded articles has previously been disclosed in U.S. patent application Ser. Nos. 08/975,729, now U.S. Pat. No. 5,945,134, and U.S. Ser. No. 08/831,129, now U.S. Pat. No. 5,868,982. In such apparatus and process, glass fiber strands comprised of a multiplicity of substantially continuous glass fibers are chopped into segments of the desired length and hydrated to a moisture content sufficient to cause the strand segments to coalesce into pellets upon tumbling. Thereafter, the strand segments are subjected to a first tumbling action to distribute the hydrating solution substantially uniformly over the strand segments and to cause the strand segments to combine to form pellets. The density of the pellets is then increased by compacting the pellets by a second tumbling action. This process can be performed by an apparatus comprising: (a) means for cutting the glass fiber strands to form chopped strand segments; (b) means for conveying the chopped strand segments to a first tumbling means; (c) means for applying a hydrating solution to the chopped strand segments; (d) a first tumbling means for imparting a tumbling action to the chopped strand segments to disperse the hydrating solution and cause the chopped strand segments to align and coalesce into pellets; (e) means for conveying the pellets to a second tumbling means; (f) a second tumbling means for tumbling the pellets to compact them and increase their density; (g) means for conveying the densified pellets to a dryer; and (h) a drying means adapted to receive and dry the pellets.
While such a process and apparatus provide numerous advantages in the preparation of chopped glass fiber pellets for use as reinforcement in molded polymer compositions, such pellets may still experience degradation during processing, storage and handling prior to compounding. Such degradation may result in pellets breaking open prematurely, resulting in the release of filaments or fuzz that can accumulate and block or impede the flow of pellets through conveyors or processing equipment. Moreover, such degradation may result in actual breakage of fibers thereby causing a reduction in the average length of the fibers in the composite article, and a consequent reduction in the physical properties of the composite article.
Accordingly, a need remains for a means of imparting greater impact resistance and toughness to the resulting pellets to reduce the degradation such pellets experience during storage and handling prior to compounding and molding. Such a need is fulfilled by the invention described in detail below.
According to the present invention, the exterior surface of the glass fiber pellets are coated with a polymeric binder composition, which, upon setting, hardening or curing (hereinafter referred to collectively as xe2x80x9ccuringxe2x80x9d), imparts increased structural integrity and toughness to the resulting pellets. The substantial encapsulation of the pellets in the cured binder improves the ability of the pellets to be stored and transported with reduced pellet degradation. Additionally, the presence of the binder coating on the pellet surface allows the pellets to travel further through the extruder during compounding before dispersion of the bundle occurs. This typically results in an increase in the average fiber length of the reinforcing fibers in the composite, which usually results in better physical properties. Further, because of the presence of this exterior coating, the pellets can be formed from strands with reduced binder loadings and corresponding lower strand integrity, which provides for quick dispersion of the fibers once the outer shell is broken. Moreover, because the binder is being applied to the outside of the formed pellets, the quantity of binder required to provide the desired integrity is typically lower than that which would be required if the binder was applied to the individual strands prior to or during pellet formation. Consequently, applying the binder to the pellet surface can reduce the overall percentage of binder in the pellets, which provides obvious economic benefits.
In the process of the invention, the pellets are coated with a curable binder composition after their formation, which, after curing, provides a glass fiber pellet that is substantially encapsulated in a thin film or shell of polymeric binder. Such pellets exhibit enhanced toughness and ability to withstand handling with reduced degradation, yet disperse quickly during compounding once the shell is broken. If the pellets are oven-dried, it can be advantageous to apply the binder to the pellets as they exit the drying oven. Applying the binder in this manner permits use of the heat contained within the pellets exiting the oven to evaporate any solvents or liquid carriers in the binder composition, which obviates the need for a secondary drying process to dry the binder, and helps cool the pellets for storage or packaging. Additionally, if the binder is heat-curable, the heat contained within the pellets can cure the binder. Alternatively, a light-curable binder may be applied to the pellets after they are dried which can be cured by passing the binder coated pellets through a suitable light chamber to cure the binder composition prior to storage or packaging.